The performance of gas turbine engines, whether measured in terms of efficiency or specific output, is improved by increasing the turbine gas temperature. It is therefore desirable to operate the turbines at the highest possible temperatures. For any engine cycle compression ratio or bypass ratio, increasing the turbine entry gas temperature produces more specific thrust (e.g. engine thrust per unit of air mass flow). However, as turbine entry temperatures increase, it is necessary to develop components and materials better able to withstand the increased temperatures.
This has led to the replacement of metallic shroud segments with ceramic matrix composite shroud segments having higher temperature capabilities. To accommodate the change in material, however, adaptations to the segments have been proposed. For example, EP 0751104 discloses a ceramic segment having an abradable seal which is suitable for use with nickel base turbine blades, and EP 1965030 discloses a hollow section ceramic seal segment.
A difficulty with ceramic shroud segments is their typically lower thermal expansion coefficient relative to the metallic parts of the engine. Differential thermal mismatches can make fixing of the segments to the engine problematic and can lead to unacceptable loadings on the segments.
A further difficulty, particularly with ceramic matrix composite shroud segments, is configuring the segments in a way that is compatible with composite forming techniques.